The present invention relates to a method for joining polyolefin pipes and a treatment for joining polyolefin.
Heretofore, electrofusion method (EF method) have been employed for joining polyolefin pipes (the pipes made of polyethylene, polypropylene, polybutene, polymethylpentene and the like) for conducting gas, waterworks, hot-water heating system, etc. In this method, the polyolefin electrofusion joint (EF joint) 3 wherein a spiral electric heating wire 1 is embedded near its inner surface when produced, as shown in FIGS. 1 and 2, and two polyolefin pipes 5 and 7 to be connected are set together, and then a specific amount of electrical energy is supplied from a controller (not shown) to the electric heating wire 1 to generate heat, whereby the inner surface of the EF joint 3 and the outer surfaces of the polyolefin pipes 5 and 7 are fused and welded.
The EF method is rapidly spreading because of the following advantages: 1) it is readily operable that the pipes can be joined and repaired with ease after being cut; 2) the operation can be automated to give stable installation quality in the field; 3) tools for this method are available and can be maintained at a low cost; 4) EF joint, although being relatively expensive, can reduce the total cost of installation, etc.
Prior to joining polyolefin pipes by the EF method, the surfaces of the pipes are preliminary planed (scraped) and wiped with an organic solvent such as acetone and alcohols to form a good fusion-jointed portion. By these pretreatments, oxides (usually about 20 xcexcm) and deposits (sand, muddy water, machine oil, grease, minute polyolefin particles produced by planing) on the surfaces of the polyolefin pipes is removed. As a result, the joint between the EF joint and the pipes is given a favorable condition.
However, joining polyolefin pipes by the EF method is often carried out in the field wherein favorable operating conditions are unlikely. Thus, the situations which impair the favorable fusion-joining are likely to occur after the completion of the above-mentioned pretreatments. A typical example of such situations is a sand particle deposited on the surfaces of the polyolefin pipes or the EF joint. This sand deposit is often invisible to the naked eye, and therefore may remain deposited when the EF joint and polyolefin pipes are jointed by the EF method. It is confirmed that the joining carried out in such situation has little problem under normal conditions. However, when an extraordinary force as by an earthquake is applied to the joining portion, a crack may occur at an incompletely fused portion of the joint portion, leading to hazards such as gas leakage.